Silicon heterojunction solar cells: Recent technological development and practical aspects - from lab to industry
Introduction
In recent years, an increasing number of silicon solar cells were reported that feature energy conversion efficiencies greater than 25% [1], [2], [3], [4], [5], [6]. One key element that these solar cells all have in common is that passivating contacts are used for charge carrier collection. Such contacts enable high efficiencies through the reduction of recombination by the displacement of the metal contact from the silicon surface. One possible approach is the use of silicon heterojunction (SHJ) contacts formed by the deposition of hydrogenated amorphous silicon (a-Si:H) layers on the surfaces of the silicon absorber. Combining intrinsic a-Si:H layers (a-Si:H(i)) that provide excellent defect passivation at the silicon surface in stacks with p- or n-doped a-Si: H, enables the formation of selective and passivating contacts. The achievement of fill-factors (FF) well above 80% has been for long a challenge for SHJ solar cells in both academia and industry, while this was not the case for homojunction solar cells. Kinoshita et al. of the company Sanyo Corp. (now Panasonic Corp.) were the first to publish a FF above 80% in 2011 [7]. As can be seen from Fig. 1, at that time this was still below the FF of the long-lasting world record obtained on a laboratory Passivated Emitter and Rear Contact1 (PERC) solar cell (FF = 82.9% [8]). It took until 2013 that Taguchi et al. from Panasonic Corp. published a SHJ solar cell with a FF of 83.2% [9], exceeding the FF of the laboratory PERC cell. The efficiency of the cell reported by Taguchi et al., however, was still below 25% as a result of a relatively low short-circuit current (). The most common approach to attain the highest possible of solar cells is to place both carrier collecting contacts at the rear side of the solar cell, avoiding both shadowing by the metal contact grid as well as parasitic absorption in the front contact layers. The latter is specifically limiting two-side contacted SHJ solar cells, due to the high absorption coefficient of a-Si:H in the visible spectrum. We discuss this issue ( for two-side SHJ) in Section 2.3.1.
A challenge for back contacted SHJ solar cells is that only half of the wafer surface is available for contact formation. In combination with the general challenge to obtain low-ohmic contacts with SHJ this explains the reduction in FF of the IBC-SHJ solar cell presented by Masuko et al. [2] compared with the previous SHJ record [9]. Still, the application of an all-back-contact architecture led to an increase in and with an efficiency of 25.6% set a new record for c-Si solar cells in 2014 [2].
In March 2017, Kaneka Corp. published their work on IBC-SHJ with the first silicon solar cell exceeding 26% efficiency [10] with a FF of 83.8%. This high FF was enabled by a series resistance of only 0.32 Ω cm2, demonstrating that very low-resistive contacts can be achieved also with SHJ contacts. Later in 2017, further progress in efficiency was reported, culminating at 26.7% [4]. This cell featured an even higher FF of 84.9%, enabled by its very low series resistance of only 0.2 Ω cm2 [1]. To reach such high FF, not only transport losses have to be minimal but also recombination in low injection conditions, both in the silicon absorber as well as at its surfaces, needs to be sufficiently low [11]. These aspects were not covered in previous review articles [12], [13]. The impact of recombination in the silicon absorber on the FF was covered by Leilaeioun & Holman, but the surface recombination was not considered in this paper [14].
Therefore, in this review, we put focus on recent progress of the FF in SHJ solar cells. After introducing possible SHJ device architectures in Section 2.1, we discuss the prerequisites to reach high FF with the help of the simulation of implied JV characteristics considering recombination both in the absorber and at its surfaces in Section 2.2.2. In Section 2.3 we review the loss mechanisms affecting the , , and FF of SHJ devices, including resistive losses into our calculations (Section 2.3.3). The impact of different interconnection technologies as well as binning of cells with slightly different JV characteristics on the FF of a module is discussed in Section 3, while possible challenges for mass production are covered in Section 4.
Section snippets
SHJ solar cell devices
Silicon heterojunction solar cells consist of a crystalline silicon wafer that is passivated on both sides with stacks of intrinsic and doped hydrogenated amorphous silicon (a-Si:H) layers. As the conductivity of intrinsic a-Si:H is very low, its thickness should be as low as possible, but a minimum thickness has to be retained to provide sufficient surface passivation (cf. Section 2.2.1). If at the front side,2 also the thickness of the doped a-Si:H
SHJ solar modules
Several companies (e.g. Panasonic Corp., Meyer Burger) already demonstrated that the good performances of SHJ solar cells can be translated into highly efficient solar modules. An important metric for module manufacturers is the so-called cell-to-module (CTM) power ratio which is the module power divided by the sum of the power of its constituent cells. CTM power ratios can vary greatly depending on module type and features, e.g. white backsheet modules benefit from a ≈ 2% gain in short circuit
Industrial aspects
Sanyo Corp. (now Panasonic Corp.) was the first company to commercialize SHJ solar cells, and produced for many years some of the most efficient c-Si modules with the lowest temperature coefficient [111]. In the last few years, several companies have launched pilot production, or even mass production of SHJ solar cells. Some companies recycled parts of the equipment s designed for the production of thin film silicon solar cells for depositing some of the SHJ layers. In parallel, several
Summary
Silicon heterojunction (SHJ) solar cells are part of the family of solar cells with passivating contacts; they feature high open-circuit voltages (), generally well above 700 mV. Achieving of fill-factors (FF) comparable to the best high-efficiency devices based on homo-junctions has long been a challenge for SHJ solar cells. In 2017, Kaneka Corp. demonstrated a SHJ solar cell with interdigitated contacts at the rear side with a FF of 84.9%, the highest ever shown for a silicon solar cell so
Acknowledgements
The authors gratefully acknowledge the support of the Qatar Foundation for funding. Raphaël Monnard and Gabriel Christmann, for support in developing the python script, Jacques Levrat for help with the CTM simulations, and Peter Fiala for proofreading abstract and summary. This project has received funding from the European Union's Horizon 2020 research and innovation programmes under Grant Agreements N°727529 (Disc), N° 727523 (NextBase), and N° 745601 (Ampere).
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